Thanks for your words, Jaysmum. This morning Fraser was let out in the garden and again retreated to the far end into the shadows of a bush. He's been doing that since it all started last Friday. He sits hunched looking at me and has to be lured out. Then he comes and eats his breakfast, no problem. While he was waiting for me to make his breakfast, this morning he sat in another bush opposite the patio door watching me. Usually he'd be hovering around my feet urging me on. He's strong in his body which I can see by the spring in his trot when out for a walk, but his behaviour otherwise is so low and out of character. Yesterday evening when I was putting his harness on, my hand brushed his muzzle and he let out a loud yelp ending in a whimper. ?? It was such a small, light contact. He can open his mouth to eat, pant, even pick up dry kibble I throw into the grass for him to search out (corn kernel sized) but doesn't open his mouth for larger objects or to yawn. The weather here has been mild, but not hot as you're having in the UK, so I don't think the bushes retreat has to do with being too warm though he occasionally opens his mouth to pant a bit while we're out walks in the direct sun. It's a huge mystery that my mind cycles through trying to find a clue that might explain what's going on. Keeping a diary is a good idea though my posts here have been useful as a record over time.
In the meantime, he's on gabapentin and tramadol (the gabapentin was added by the second vet who saw him on Sunday) on the basis of treating his symptoms since she couldn't see what could be causing pain. I've essentially got my second opinion, so I don't suppose it's any use spending more money to go to a third vet. I guess I'll wait for a while longer to see if there's any signs of his...what?...I don't even know what it is...mouth issue subsiding. If it doesn't improve, I'll have to consider going into Vancouver for a deeper look. That was already on the cards for investigation of his IBD that I'd been delaying because of Covid (I'm in the vulnerable oldster group), but now I'm wondering if I'll just have to do it and get the mouth thing checked as well.